1. Field of the Invention
The present invention relates to a fuel injection control system for an internal combustion engine. More particularly, the present invention relates to a fuel injection control system in an internal combustion engine in which fuel injection valves have been arranged on the upstream and downstream sides of a throttle valve.
2. Description of Background Art
When a fuel injection valve is provided upstream from the throttle valve, the volumetric efficiency is improved because heat is taken from intake air when injection fuel vaporizes. Therefore, the engine output can be increased as compared with when the fuel injection valve is provided downstream from the throttle valve.
However, when the fuel injection valve is provided on the upstream side, a response lag occurs in fuel transport as compared with when the fuel injection valve is provided downstream from the throttle valve because a distance between the fuel injection port and a combustion chamber inevitably increases.
Japanese Patent Laid-Open Nos. 4-183949 and 10-196440 discloses a fuel injection system in which fuel injection valves have been provided in the intake pipe at locations upstream and downstream from the throttle valve in order to improve the engine output and cope with the response lag.
FIG. 7 is a cross-sectional view showing a major portion of an internal combustion engine according to the background art, in which two fuel injection valves have been arranged with the throttle valve 52 of the intake pipe 51 interposed therebetween. Specifically, a first fuel injection valve 50a has been arranged on the downstream side and a second fuel injection valve 50b has been arranged on the upstream side.
In an internal combustion engine equipped with such fuel injection valves, an accelerated increase in quantity and correction has been experienced, in which an accelerated driving state is detected to increase and correct an injection quantity of fuel. During such accelerated increase in quantity and correction, it is necessary to control an air-fuel ratio in such a manner that fuel does not lag behind an increase in intake air quantity during acceleration. Accordingly, it is requested to quickly increase the quantity of injection fuel. In view of this, in an internal combustion engine equipped with two fuel injection valves, if the fuel for an accelerated increase in quantity and correction is dispersed into both the upstream side and the downstream side, it will becomes difficult to obtain sufficient acceleration performance due to a response lag which occurs with the injection quantity on the upstream side.